Atkins — The H/jilrogcn Ion Concentration of Plant Cells. 415 



No further emphasis need be laid upon the desirability of precise 

 information on a factor so intimately concerned with the regulation of many 

 processes of the cell. 



As far back as 1902 Friedenthal applied the method of measuring 

 electrical potential differences to the determination of the reaction of 

 animal liquids. Its application to plant physiology came much later, though 

 mentioned by Friedenthal (1910). But from 1916 onwards a number of 

 determinations have been made, such as those of Haas (1916, '17, 19, '20); 

 of Bunzel (1916) ; Eeed (1916, 1 and 2) ; Hempel (1917); Kappen (1918) ; 

 Patten and Mains, quoted by Clark and Lubs (1917); Truog and Meacham 

 (1918); Fred and Davenport (1918); Clevenger (1919); Hoagland (1919) ; 

 Swanson and Tague (1919). The I'esults of some of these researches will be 

 mentioned further on. 



Hydrogen ion concentrations met with in pla^its. 



From early work on the subject it was established that plant cells might 

 be neutral or slightly acid to litmus, and the rather strong acidity of unripe 

 fruits is well known. Hof (1909) used iodo-eosin as a test for free alkali in 

 dried-up plant tissues ; but results thus obtained are, it seems, highly 

 misleading if taken to apply to the living plant. 



The numerous electrometric and colorimetric researches already men- 

 tioned showed that plant-sap ranges from almost neutrality to pH 1-7, 

 the value for lime juice, as given by Patten and Mains. Values for other 

 fruits ranged from pH 4-5 downwards, viz. in the direction of greater acidity ; 

 but a lesser degree of acidity is met with in massive storage organs, such 

 as potato tuber pH 6'2, sweet potato pH 5'7-6'6, beet pH 6-6. For those 

 unaccustomed to think of acidity in pH values it may be of use to mention 

 Wherry's (1919) suggested nomenclature for soil studies ; this author takes 

 the hydrogen ion concentration of pure water as unity. For this pH = 7 ; 

 for a solution of acidity pH = 6, the "specific acidity" is 10, because 

 the concentration in hydrogen ions is ten times as great as in pure water. 

 Thus it may be seen that the "specific acidity" of lime juice, taking it as 

 pH = 2, is as great as 100,000. It is 10,000 times more acid than potato 

 juice. An even higher value, pH = 1'4, was found by the writer for an 

 Indian berry, something like a gooseberry in appearance. For comparison 

 it may be mentioned that the value for acentinormal solution of hydrochloric 

 acid is pH = 2 ; yet this is usually considered a very dilute acid. Eor 

 adult gastric juice pH = 1-8 approximately, and a solution of carbon dioxide 

 in water nearly saturated at 25° C. is at pH = 4'8. On the other side the 



